1. Acta Astronautica ] (]]]]) ]]]–]]]
Contents lists available at SciVerse ScienceDirect
Acta Astronautica
journal homepage: www.elsevier.com/locate/actaastro
Searching for alien artifacts on the moon
P.C.W. Davies a,n, R.V. Wagner b
a
Beyond Center for Fundamental Concepts in Science at Arizona State University, PO Box 871504 Tempe, AZ 85287, USA
b
School of Earth and Space Exploration Lunar Recon Oribiter Sci Ops Cn, Arizona State University, Tempe, AZ, 85287, USA
a r t i c l e i n f o abstract
Article history: The Search for Extraterrestrial Intelligence (SETI) has a low probability of success, but it
Received 5 March 2011 would have a high impact if successful. Therefore it makes sense to widen the search as
Received in revised form much as possible within the confines of the modest budget and limited resources
7 October 2011
currently available. To date, SETI has been dominated by the paradigm of seeking
Accepted 29 October 2011
deliberately beamed radio messages.
However, indirect evidence for extraterrestrial intelligence could come from any
Keywords: incontrovertible signatures of non-human technology. Existing searchable databases
SETI from astronomy, biology, earth and planetary sciences all offer low-cost opportunities
Lunar reconnaissance orbiter
to seek a footprint of extraterrestrial technology. In this paper we take as a case study
Extraterrestrial technology
one particular new and rapidly-expanding database: the photographic mapping of the
Nuclear waste
Lava tubes Moon’s surface by the Lunar Reconnaissance Orbiter (LRO) to 0.5 m resolution.
Regolith Although there is only a tiny probability that alien technology would have left traces
on the moon in the form of an artifact or surface modification of lunar features, this
location has the virtue of being close, and of preserving traces for an immense duration.
Systematic scrutiny of the LRO photographic images is being routinely conducted
anyway for planetary science purposes, and this program could readily be expanded
and outsourced at little extra cost to accommodate SETI goals, after the fashion of the
SETI@home and Galaxy Zoo projects.
Published by Elsevier Ltd.
1. Background human capacity to meaningfully extrapolate regarding
its specific characteristics. If we therefore have no fixed
In 2010 the Search for Extraterrestrial Intelligence idea what to look for, it makes sense to search all
(SETI) was 50 years old, and the anniversary provided available and emerging databases for ‘‘artificiality,’’
an appropriate opportunity to take stock. In particular, whether deliberate (as in a message) or inadvertent (as
there have been calls to widen the search by considering in environmental impact). We argue that the criteria for
not just purposefully-directed radio messages, but the searching a database should be primarily tied to cost
most general signatures of intelligence [1]. Alien intelli- rather than plausibility. If it costs little to scan data for
gence, if it exists, or has existed, might have endured for a signs of intelligent manipulation [3], little is lost in doing
very long duration, measured in millions, tens or hun- so, even though the probability of detecting alien tech-
dreds of millions or even (in principle) billions of years, nology at work may be exceedingly low. A good example,
and will almost certainly be dominated by post-biological already discussed in the literature [4,5], is genomic
intelligent systems [1,2]. Thus it may lie far beyond SETI—the ‘‘message-in-a-bottle’’ scenario in which an
extraterrestrial civilization long ago uploaded a message
into the DNA of some terrestrial organisms, either roboti-
n
Corresponding author. cally or using viral vectors. Genome sequencing is taking
E-mail address: deepthought@asu.edu (P.C.W. Davies). place on a grand scale anyway, the results freely available
0094-5765/$ - see front matter Published by Elsevier Ltd.
doi:10.1016/j.actaastro.2011.10.022
Please cite this article as: P.C.W. Davies, R.V. Wagner, Searching for alien artifacts on the moon, Acta Astronaut. (2011),
doi:10.1016/j.actaastro.2011.10.022
2. 2 P.C.W. Davies, R.V. Wagner / Acta Astronautica ] (]]]]) ]]]–]]]
on the internet, and it would cost almost nothing to behind by an alien expedition or colonization wave, to
search the genomic database using a simple algorithm record data, perhaps about the development of life on
to seek out signs of intelligent manipulation. Earth, over a long period of time, and may or may not
In this paper we discuss a new database resource: the still be active.
photographic mapping of the lunar surface to a resolution of 3) Trash. Objects left behind by an alien expedition or
0.5 m, currently being conducted by the Lunar Reconnais- colonization wave, without any regard to whether or
sance Orbiter (LRO). (For a review of the mission in general, not they survived intact. These could range from
see Chin et al. [6], and of the camera system in particular, landing stages of spacecraft to spent radiogenic or
see Robinson et al. [7]). A persistent science fiction theme is other power sources.
that of an alien artifact left on the moon’s surface long ago 4) Geo-engineering structures. Not artifacts per se, but
(for example, Clarke [8]). Such an artifact might originate in changes to the moon’s surface caused by mining,
several ways, for example, discarded material from an alien quarrying or construction activities.
expedition or mining operation, instrumentation deliber-
ately installed to monitor Earth, or a dormant probe await- A key issue is when such a probe might have been sent,
ing contact (a variant on the message-in-a-bottle theme). or a visitation may have taken place, because that will
Alien technology might also manifest itself in mining or affect the detectability of any traces. Studies of extra-solar
quarrying activity, or even construction work, traces of planets suggest that earthlike planets may be common in
which might persist even after millions of years. Whatever the galaxy [12]. The galaxy is about 13 billion years old,
may be the reason for alien artifacts or engineering pro- but the solar system formed only about 4.5 billion years
cesses on the moon, the question that we address here is ago. If we restrict attention to carbon-based life, then the
whether we could in principle detect such a thing using LRO probability for life to emerge will slowly rise as a function
data or similar foreseeable surveys. of the availability of carbon and related elements. An
The moon has several factors in its favor as a place to estimate of the age distribution of terrestrial planets has
search for alien artifacts. First, it is close. Instruments based been given by Lineweaver [13]. Of course, we have no idea
on Earth can observe the surface in reasonably high detail, what the absolute probability is for life to arise on a
and instruments on the moon can communicate with Earth terrestrial-type planet once it exists. It could be very rare
at high bandwidths. Second, it is largely unchanging. The only [14,15], but for the purposes of this paper we shall adopt
major source of erosion and deposition comes from meteorite the optimistic position of Davis and Lineweaver [16] that
impacts, which occur at a very slow rate (the seismometer on the probability for abiogenesis it is not exceptionally low,
Apollo 12 detected only one grapefruit-sized impactor a and that there are consequently many planets with life in
month within a 350 km radius). The process of ‘gardening,’ our galaxy (and others). In spite of the steady increase
whereby continual disruption of the lunar surface by impacts over time of the relative probability for life to emerge,
leads to eventual burial, has been studied in the context of there is no reason to rule out the possibility that planets
locating terrestrial meteorites on the moon [9]. Calculations with life existed long before Earth formed, and that
indicate that it could take hundreds of millions of years for an planets with intelligent life and technological commu-
object a few tens of meters across to vanish by this process nities existed, and have existed, over astronomical time
beneath the dust and regolith [10,11]. However, gardening scales [17,1]. Thus there is no reason to suppose that an
also serves to re-expose buried objects over a long period of alien probe would have been sent to the solar system, or
time. Third, the moon is tectonically inactive, so any activity an alien visitation taken place, in the relatively recent past
caused by an artifact (heat, strong magnetic field, radio- (e.g. within the epoch of human habitation on Earth), as
opposed to the very far past. If the solar system were
activity, etc.) will show up clearly, rather than being hidden
visited, say, two billion years ago, it may be exceedingly
by other processes such as might occur on Earth. Thus any
difficult to identify any traces of alien technology even on
messages or traces of technological activity left on the moon
a fairly large scale. If the time scale was, say, one hundred
will likely still be around today, and should be possible to
million years, there is more likelihood of us finding traces,
locate without the vast expense that would go into searching
but our best hope is if we are dealing with a time scale of
more distant bodies, such as Mars. The question, then, is
a few million years or less.
what forms these artifacts might take, and why they might
have been left.
There are many conceivable reasons that aliens would 2. Messages
leave artifacts on the moon, including some that humans
may not even be able to guess at. If for the sake of argument This is the most attractive possible artifact type to find,
we assume an alien species with roughly human-like reason- as it shows that not only does alien intelligence exist, but
ing and motivation, there are four main classes of artifacts or that it cares (or cared) about communicating with other
technological modifications that we should consider intelligent beings, and may even be willing to impart
knowledge and wisdom to us. A message would likely be
1) Messages. These are artifacts specifically designed to be placed where it could be easily located, and might also
found and interpreted by an intelligent species that have some sort of beacon attached, so that orbital instru-
developed on Earth. ments would be able to find it even if the message capsule
2) Scientific instruments. These are observational devices was buried. However, the details of these features can
sent across interstellar space as robotic probes, or left vary widely depending on when the putative aliens
Please cite this article as: P.C.W. Davies, R.V. Wagner, Searching for alien artifacts on the moon, Acta Astronaut. (2011),
doi:10.1016/j.actaastro.2011.10.022
3. P.C.W. Davies, R.V. Wagner / Acta Astronautica ] (]]]]) ]]]–]]] 3
visited, and how long they expected the message to need might be just a few tens of meters across. Furthermore, an
to last. instrument would likely be coated in lunar dust after a
If aliens (or their probes or robotic surrogates) visited few hundred years, making it had to distinguish from a
in the last few thousand years, they would know that mundane geological feature. The best way to identify an
there was a developing technological society on Earth, active instrument would probably be to look for signs of
and they might leave a simple capsule in or near an its power source. Solar panels, if kept clean enough to
existing landmark, such as a large fresh crater like Tycho, keep the instrument powered, would probably appear as
perhaps with a radio beacon or a splash of paint to further large black geometric shapes. Radiogenic sources would
mark its location. In this case, the message could probably likely produce detectable gamma radiation, as there is
be spotted by orbital photography such as LRO. Messages little incentive for the aliens to go to the effort of shielding
designed to last only a few million years would probably the reactor. More exotic power sources might also pro-
be similar, although any beacon would need to be detect- duce detectable radiation, although we cannot currently
able through a few centimeters of regolith. However, predict what these might be, so there is no good way to
messages designed for such a timescale might be unlikely actively search for them.
unless the aliens could have predicted the eventual rise of Of the above, solar is the easiest to find with current
technological civilization based merely on the discovery data, such as from the LRO, and there is an obvious place
that Earth hosted one or more species of big-brained tool- to look for solar arrays. At the north and south poles, there
making animals. are regions that are in near-perpetual sunlight, usually
In the case of a message designed to last hundreds of very close to permanently-shadowed craters that could be
millions of years all bets are off. Over such a time scale, used to hold instruments that require very low tempera-
any given point is likely to be hit by a meteorite large tures, such as infrared telescopes. Thus, looking at high-
enough to destroy artifacts on the surface made out of any resolution images of these areas should reveal whether or
known material, so to survive for 4108 years the mes- not there are alien solar arrays installed. However, there is
sage would need to be securely buried at depth. There- very good high-resolution coverage of the lunar poles
fore, it would also need a beacon that is detectable thanks to the LRO NAC camera, and no obvious solar
through at least several meters of regolith. This could be arrays have been found so far.
long-wavelength radio, a strong magnetic field, or (in
principle) something more exotic such as a neutrino
generator. It could also, of course, involve some form of 4. Trash
more advanced technology as yet unknown to us. In any
case, there would probably be no visual evidence left on Any alien expedition that set down on the moon is
the moon’s surface to attract our attention that anything likely to have left some trash behind, as it is expensive to
of interest was in the area. However, there might still be a haul material out of any gravity well, even one as shallow
way to narrow the search. The north and south poles are as the moon’s. Aliens might not have been as wasteful as
obvious choices for placing messages, but thanks to the the Apollo missions, that left most of the equipment
moon’s tidally-locked orbit, there are other points that behind, along with half of the landing spacecraft, but they
can be uniquely pin-pointed over a long period of time, might still have left something. Finding small artifacts
such as the sub-Terran point (the point directly below the such as dropped tools is probably hopeless, but larger
Earth) and its antipode, and the centers of the leading and structures such as habitat domes or solar arrays might
trailing hemispheres in the moon’s orbit. One of these still be visible millions of years later.
might be deliberately chosen as the site of a message in One particular form of trash – spent radioactive fuel –
the knowledge that these points would have significance is an attractive target. Nuclear waste may contain radio-
to a scientifically literate terrestrial community. But with- nuclides with very long half lives, leaving traces that are
out any surface features to indicate the presence of a detectable for millions of years or even longer. (The Oklo
subsurface artifact, detection would have to depend on natural nuclear reactor discovered in Gabon, West Africa,
ground-penetrating radar or deliberate excavations by went critical about 2 billion years ago, and has been
future manned expeditions. intensively studied; see, for example, Petrov et al. [18])
Any deposits of plutonium, a good nuclear fuel, would
3. Scientific instruments stand out as a product of alien technology because the
half life of this element is much less than the age of the
These would be nearly as good as an actual message if solar system, so any primordial plutonium on the moon
we found them, as they would not only prove the would have decayed long ago. If a nuclear waste dump
existence of alien intelligence, they would tell us some- were found, it should be possible to date both the age of
thing about what the beings were interested in, and the fuel and the time that it was discarded. Nuclear
possibly allow us to communicate with them if they are material left on the surface of the moon would eventually
somehow monitoring the instruments. However, finding become buried, but for a long time it would still produce a
instruments would be much harder than finding a mes- distinctive signature as a radiation source (assuming it
sage, as the aliens would have no incentive to make an was unshielded) that could readily be detected from orbit
instrument easy to spot. Even a large radio telescope on on a future mission suitably equipped to search for
the far side of the moon (to shield it from potential radio gamma rays. However, it is unlikely that the LRO could
traffic from any future terrestrial technological society) assist in searching for nuclear waste, even if it is not
Please cite this article as: P.C.W. Davies, R.V. Wagner, Searching for alien artifacts on the moon, Acta Astronaut. (2011),
doi:10.1016/j.actaastro.2011.10.022
4. 4 P.C.W. Davies, R.V. Wagner / Acta Astronautica ] (]]]]) ]]]–]]]
buried, given that it would probably be small and searching for. For alien artifacts, we don’t have the luxury
inconspicuous. of knowing what latitude and longitude to target, so we
A good place to look for alien trash is inside one of the need to study the entire surface. Focusing on some
lava tubes located in the lunar maria. So far, three large regions of special geological interest would help, but
skylights have been discovered by the LRO, each about there would still be hundreds of images to look through.
100 m across, which might lead down into a subsurface The huge amount of data from the NAC is both its best
network, and several lunar pits point to a subsurface feature and the greatest obstacle to using it as a resource
labyrinth [19]. Lava tubes have been proposed as an ideal for seeking alien artifacts. Each NAC frame is 500 mega-
location to establish a human base, as they would provide pixels, and takes between 30 min and an hour to analyze
protection from radiation and meteorites; perhaps aliens in sufficient detail to be able to locate artifacts such as the
would come to the same conclusion. Furthermore, the Surveyor landers. So far more than 340,000 NAC images
same factors that make lava tubes attractive as a habitat have been released to the public, and that number will
imply that any artifacts left behind would endure almost likely approach 1,000,000 by the time it has achieved 100
indefinitely, undamaged and unburied. The downside is percent coverage. From these numbers, it is obvious that a
that there is no way to really investigate this possibility
manual search by a small team is hopeless. It might be
from orbit, so any confirmation or refutation will require a
possible to scan the entire dataset by ‘‘crowdsourcing’’ the
new robotic or human mission to the surface.
work to a few tens of thousands of people over the
Internet, and in fact this approach is being taken by the
5. Geo-engineering structures
website MoonZoo.org, which is trying to classify as many
features on the moon as possible by having people look at
If aliens used lunar material as a resource, they may
have carried out mining or quarrying activities, or even a few NAC frames each, and record what they find. The
have built large structures that could still be detected downside to this approach is that with so many people
from photographic surveys. The main difficulty in identi- involved, there will be differences in opinion on what is
fying the scars of major geo-engineering work would be and isn’t important. Without a lot of organization and
to distinguish them from naturally occurring features. A dedication, many images will get skipped and many
round open-cast mine, for example, may after some potentially interesting features overlooked. Nevertheless,
millions of years come to resemble an impact crater or this project will doubtless result in some claims of alien
collapsed lava tube at first sight, and only a careful artifacts, and one or more of them might conceivably be
analysis of the topography might reveal signs of artifici- correct.
ality. Excavations with more distinctive topography (spir- An alternative method of searching a large number of
als, rectangles, etc.) would be more conspicuous. Because images is computer automation. One of us (RW) has
we have no idea of the motives, capabilities or agenda of a written software able to identify simple pits as part of
very advanced alien technological community, we cannot the in-house LRO data analysis program currently being
guess what form of surface modification might ensue conducted at Arizona State University. This system is able
from an alien presence, even a fleeting one, on the moon. to run through about 200 images per hour. It merely
It therefore pays to be as broad as possible when seeking filters the images, identifying the features most likely to
signs of past geo-engineering activity. be pits, and presents them for a human analyst to decide
whether the features are interesting or not. However, it
6. Seeking artifacts using current data does allow a human to search thousands of images each
day for a specific feature.
The best available visible-light imagery of the moon Although automated searching offers great promise, a
comes from the Narrow Angle Camera (NAC) on the Lunar major obstacle is that computer software can only search
Reconnaissance Orbiter, which has been orbiting the for specific features that have been programmed in
moon since mid-2009 [7]. It has imaged over 25 percent advance. Thus, for example, crater detection algorithms
of the lunar surface at resolutions down to 50 cm/pixel, in are now fairly sophisticated [20,21]. However, when it
a variety of lighting angles. This dataset is so good that comes to ‘‘artificiality’’ there is inevitably an element of
several artifacts have in fact already been found, in both judgment involved at the outset as to what would con-
the Instrument and Trash categories. However, all of them stitute evidence. Certain features, such as regular geome-
were created by humans. These artifacts include not only trical shapes or sharp angles, are relatively easy to deal
the Apollo landing sites, which are easily identified by the with, but more subtle traces, for example, partially buried
thin dark trails of dust kicked up by the astronauts, but smooth surfaces or quarry boundaries, offer a greater
also all of the NASA and Soviet unmanned probes, which, challenge. As any forensic scientist can attest, physical
with the exception of the two soviet rovers that left evidence for intervention by an intelligent agent may be
kilometers-long tracks, have nothing to mark their loca- very subtle and require multiple lines of evidence and a
tion but their slightly odd-looking shadows, and some- lifetime of experience for it to become apparent. Because
times a small halo of disturbed dust from the landing we have no clear idea of what to look for, the task is
rockets. However, in all of these cases, the people who doubly difficult. Nevertheless, the NAC data is being
found the artifacts already knew approximately where to gathered anyway, and the cost of searching this
look, so they only needed to comb through a few NAC amazing resource, either by eye or by software, is rela-
images before they found the lander or rover they were tively modest.
Please cite this article as: P.C.W. Davies, R.V. Wagner, Searching for alien artifacts on the moon, Acta Astronaut. (2011),
doi:10.1016/j.actaastro.2011.10.022
5. P.C.W. Davies, R.V. Wagner / Acta Astronautica ] (]]]]) ]]]–]]] 5
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Paul Davies is Director of the Beyond Center
greatest scientific discoveries of all time. for Fundamental Concepts in Science at Ari-
zona State University. His research spans
cosmology, astrobiology and theoretical phy-
Acknowledgments sics. He has made important contributions to
quantum field theory in curved spacetime,
with applications to inflationary cosmology
We should like to thank Chris McKay and Mark and black holes. He was among the first to
Robinson for helpful discussions. champion the possibility that microbial life
could be transferred between Mars and Earth
in impact ejecta. He also runs a major cancer
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Cancer Institute. He is the author of 28
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Please cite this article as: P.C.W. Davies, R.V. Wagner, Searching for alien artifacts on the moon, Acta Astronaut. (2011),
doi:10.1016/j.actaastro.2011.10.022